Prince Philip is back in hospital after a recurrence of a bladder infection, Buckingham Palace said, the third time he has been admitted to hospital in less than a year.
The Duke of Edinburgh fell ill while staying with the Queen at Balmoral, their Scottish royal residence, and he was driven the 40 miles (60km) from there to Aberdeen Royal Infirmary.
"The Duke of Edinburgh has had a recurrence of the bladder infection that he had earlier in the summer and has been admitted to hospital for investigation and treatment," a Palace spokesman said.
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"The prince is likely to remain there for the next few days."
Despite recent illnesses, the sprightly 91-year-old Duke has continued to make public appearances, notably at events held to mark the Queen's 60th year on the throne this year.
The BBC said his last public engagement had been at Cowes in the Isle of Wight off southern England earlier this week.
During the London Olympics, he accompanied the Queen to the opening ceremony and also joined other members of the royal family to watch granddaughter Zara Phillips compete in an equestrian event at the Games.
He was hospitalised two months ago when he first received treatment for a bladder infection, overshadowing four days of public celebrations for his 86-year-old wife's Diamond Jubilee.
He had braved wind and rain to watch a 1,000 boat flotilla on the River Thames from a royal barge before falling ill and being unable to attend a thanksgiving service at St Paul's Cathedral.
In December last year, Philip had an operation to clear a blocked artery in his heart after suffering chest pains, and spent four days in hospital over Christmas.
Philip, the longest-serving consort in British history has won admirers for his charity work and loyal support to the Queen, but is as well-known for his sharp wit and verbal gaffes as he is for his devotion to duty.
A stray remark about "slitty eyes" during a visit to China in the 1980s became symbolic of the Greek-born former naval officer's gruff and often unguarded manner.
Son of the exiled Prince Andrew of Greece, Philip married Princess Elizabeth, as she was then, in 1947.
They met when she was 13 on a visit to the naval college where he was a cadet. Following World War Two they became engaged, and he was given the title of the Duke of Edinburgh.